Effect of organic amendments on soil organic carbon fractions, water retention, and mechanical properties in a Chinese Alfisol

Intensive agriculture and excessive land use have significantly degraded soil structure, challenging the sustainable development of global agriculture. Organic amendments have proven effective in enhancing soil structure, increasing soil organic carbon (SOC), and regulating soil water-holding capacity while influencing soil physical and mechanical properties. However, the interplay between SOC fractions, soil water-retention capacity, and mechanical characteristics remains poorly understood. This study used a 10-month incubation experiment to evaluate the effects of different organic materials—straw (ST), biochar (BR), organic fertilizer (OR), and straw biochar (SBR)—applied at two proportions (1 % and 3 %) to assess their effects on SOC fractions, water-retention capacity, and mechanical properties of Alfisols in Northeast China. The results demonstrated that the type and proportion of organic material significantly influenced SOC and its fractions, which increased significantly with the 10-month incubation period. Higher SOC and mineral-associated organic carbon (MAOC) contents correlated with improved water retention and compressive resistance, with the 3 %-BR treatment showing the most significant effects. These findings highlight the importance of considering SOC and its role in water retention during tillage to avoid compaction, especially under high soil moisture conditions.